What is the 42dB heat pump noise limit?

In England (and, with equivalent local requirements, Wales), permitted development for a domestic air source heat pump requires a calculation under the MCS Planning Standards (MCS 020) showing the predicted noise level at the nearest neighbouring dwelling's noise-sensitive façade does not exceed 42 dB(A). It's a modelled, site-specific figure based on the unit's declared sound power, distance to the neighbour, and any screening — not a simple measurement of the unit itself.

Does every heat pump installation need a noise assessment?

If you're relying on permitted development rather than a full planning application, yes — the MCS 020 calculation is one of the conditions that makes permitted development apply in the first place. A competent MCS installer produces this as standard, using the manufacturer's published sound power data for the specific model and duty point proposed.

Do terraced houses and small gardens struggle to meet the noise limit?

They can, because the calculation is sensitive to distance from the unit to the neighbour's nearest habitable room window. A tight urban plot with a boundary a metre or two from a neighbour's window gives less margin than a detached house with a large garden. It's solvable in most cases — a quieter or better-positioned model, or an acoustic enclosure, often brings the prediction back under the limit — but it can mean a smaller range of suitable units than a rural property has.

What happens if my proposed siting exceeds the noise limit?

You have three practical options: reposition the unit further from the boundary or neighbouring window, specify a quieter model or one that can run at lower duty for the required heat output, or add acoustic screening. If none of those bring the calculation under the limit, permitted development doesn't apply and you'll need a full planning application, where the council assesses noise on its own planning policy rather than the fixed MCS 020 threshold.

Is the noise limit the same in Scotland and Northern Ireland?

No. Scotland and Northern Ireland run separate permitted development legislation from England and Wales, with their own siting and noise conditions rather than an identical import of the MCS 020 42dB(A) test. Don't assume the English figure applies north of the border or in NI — check current guidance with your local authority or, in Scotland, via Home Energy Scotland.

Can a neighbour complain about noise after installation, even if it met the limit?

Yes, in theory, through the council's environmental health team under statutory nuisance law — a separate route from planning. In practice this is rare for a correctly sized, well-sited, MCS-compliant installation; most real-world noise complaints trace back to poor siting (right under a bedroom window, hard surfaces reflecting sound) or a badly fitted unit vibrating against the building, not the unit's inherent noise level.

Heat Pump Noise Rules UK: The 42dB Limit, MCS 020 & Siting Explained

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Air source heat pump outdoor unit sited well away from neighbouring windows in a UK garden, clear airflow space, timber fence boundary

In England and Wales, permitted development for a heat pump depends on a modelled noise result at your nearest neighbour's window, not just the unit's own rated sound level.

In England, and under equivalent Welsh rules, a domestic air source heat pump can only rely on permitted development — installing without a planning application — if a standardised calculation, run under the MCS Planning Standards (MCS 020), predicts that noise at your nearest neighbour's window won't exceed 42 dB(A). Scotland and Northern Ireland use their own separate rules. If your installer hasn't mentioned this calculation, ask for it before you commit to a siting position.

Two different noise numbers, easily confused

Heat pump marketing often quotes a single decibel figure — "40dB", "quiet as a fridge" — and it's easy to assume that's the number regulators check. It isn't, quite. There are two different measurements in play:

Sound power level — a fixed characteristic of the unit itself, measured under standard test conditions and published by the manufacturer (often on the MCS product certification database). This is what "40–48 dB at 1m" figures usually describe.
Predicted noise level at the receptor — what actually reaches your neighbour's window, which depends on the unit's sound power, the distance to that window, any barriers or screening in between, and the presence of other noise sources nearby. This is what the 42 dB(A) permitted development limit actually measures.

A loud unit sited far from any neighbour can pass easily. A quiet unit sited two metres from a bedroom window on a small plot can fail. Siting matters as much as the model you choose.

What MCS 020 actually calculates

MCS 020 (the current version of what was previously known as MIS 3005's noise annex) is the standardised methodology UK MCS-certified installers use to produce this figure. It takes:

The manufacturer's declared sound power data for the specific model and operating duty
The distance from the proposed unit position to the nearest noise-sensitive room of the nearest neighbouring dwelling — typically a bedroom or living room window
Any relevant screening, barriers or reflective surfaces nearby

and produces a predicted sound level in dB(A) at that neighbouring window. For permitted development in England (and under the equivalent Welsh legislation), that result must not exceed 42 dB(A). If it does, the installation falls outside permitted development and needs a full planning application, where the local authority assesses noise against its own planning policy rather than an automatic threshold.

Ask your installer for this, specifically

A written MCS 020 calculation showing the predicted dB(A) figure at your nearest neighbour's property, for the exact model and siting position proposed — not just a verbal assurance that "it'll be fine." This is standard paperwork for a compliant installer and worth keeping alongside your Boiler Upgrade Scheme documentation if you're applying for the grant.

Other permitted development conditions worth knowing

Noise is the condition that generates the most confusion, but England's permitted development class for domestic air source heat pumps carries several others: it typically allows only one unit per dwelling under permitted development, sets a maximum outdoor unit volume, restricts installation closer to a road than the part of the building nearest that road (with some exceptions), and — as covered in our listed buildings guide — excludes listed buildings and their curtilage entirely. A local authority can also remove permitted development rights for a specific area via an Article 4 direction, most commonly in conservation areas, which means even an otherwise straightforward installation needs a full application. Check your council's Article 4 register for your address before assuming permitted development applies.

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Close-up of an air source heat pump outdoor unit on an anti-vibration rubber mounting base, isolating the unit from the building structure

Vibration transmitted through the building structure is a common source of noise complaints even when the unit's airborne dB(A) figure is fully compliant.

How the rules differ across the UK

Don't assume the English 42 dB(A) figure applies everywhere — permitted development is devolved:

England — Town and Country Planning (General Permitted Development) Order 2015, Schedule 2, Part 14, Class G, with the MCS 020 noise calculation and 42 dB(A) threshold as described above.
Wales — a separate General Permitted Development Order for Wales, broadly mirroring England's approach including a noise assessment condition. Confirm the current detail with your local planning authority, since Welsh secondary legislation is amended independently of England's.
Scotland — its own permitted development legislation with different siting and noise conditions, set separately from England's MCS 020 test. Check current guidance via your local authority or Home Energy Scotland rather than assuming the English threshold transfers directly.
Northern Ireland — separate permitted development legislation again, with its own conditions administered through local councils.

Practical siting advice beyond the legal minimum

Meeting the regulatory threshold is the legal bar, but it isn't the same as a neighbour never noticing the unit. A handful of practical siting choices make a bigger difference to real-world nuisance than the headline dB(A) figure:

Avoid siting directly under or beside any bedroom window — yours or a neighbour's — even where the calculated result is compliant. Low-frequency compressor noise is more noticeable at night and in a quiet bedroom than the dB(A) figure alone suggests.
Use an anti-vibration mounting base rather than bolting directly to a party wall or shared structure. Structure-borne vibration is one of the most common sources of noise complaints, and it isn't fully captured by an airborne sound calculation.
Leave clearance around the unit for airflow. A unit boxed in tightly against walls or fencing can be forced to work harder to maintain airflow, which increases both running noise and the risk of the calculation being optimistic versus reality.
Avoid hard, reflective surfaces close to the unit — a nearby fence or wall can reflect sound toward a neighbouring property that a more open position wouldn't.
Favour a variable-speed (inverter) unit. Most modern UK heat pumps modulate output to match demand, running quietly most of the time and only approaching peak output — and peak noise — in the coldest weather, rather than cycling on and off at full power.

Comparing models on noise: what to check before you buy

If you're choosing between heat pump models, sound power figures are only genuinely comparable when they're quoted on the same basis — the same operating condition (usually a stated outdoor temperature and duty point) and the same measurement standard. A figure quoted "at 1m" from one manufacturer and "at 5m" from another will look wildly different for the same underlying unit. Ask for the sound power level in dB(A), the standard it was tested to, and the operating condition it applies to, and get your installer to run the MCS 020 figure for the two or three models you're actually choosing between at your specific siting — the marketing number rarely tells you enough on its own to compare fairly.

If a neighbour objects or complains

If you go through a full planning application (rather than permitted development), neighbours are formally consulted and can raise noise objections that the council weighs directly. If you've installed under permitted development and a neighbour complains afterwards, that sits outside the planning system — the relevant route is a statutory nuisance complaint to the council's environmental health team under the Environmental Protection Act 1990, which investigates independently of whether the original MCS 020 calculation was compliant. This is genuinely rare for a well-sited, correctly installed unit, but it's worth knowing the route exists separately from planning.

The bottom line

The 42 dB(A) figure people search for is real, but it's a modelled result specific to your property and neighbours — not a fixed spec you can read off a heat pump's brochure. Ask your installer for the written MCS 020 calculation for your exact siting, understand that Scotland and Northern Ireland run different rules, and treat good siting practice — distance, anti-vibration mounting, airflow clearance — as worth doing even where the legal minimum is comfortably met.

Compare noise-relevant siting constraints against your property type in our flats and apartments guide, and see typical unit sound levels in context in our air source heat pumps guide.